The invention is based on a fuel injector according to the definition of the species of the main claim.
An electromagnetically operable fuel injector is already known from German Laid-Open Patent DE-OS 33 14 899 in which for the purposes of electromagnetic activation an armature acts together with an electrically excitable solenoid and the stroke of the armature is transmitted by way of a valve needle to a valve closing member. The valve closing member works together with a valve seat. The armature is not rigidly attached to the valve needle, but is arranged with axial movement relative to the valve needle. A first return spring exerts pressure on the valve needle in the closing direction and thus holds the fuel injector closed when the solenoid is non-current-bearing and thus not excited. The armature is pressed by a second return spring in the stroke direction such that in its idle position the armature is touching a first stop provided on the valve needle. When the solenoid is excited, the armature is pulled in the stroke direction and by way of the first stop takes the valve needle with it. When the current exciting the solenoid is switched off, the valve needle is accelerated to its closed position by the first return spring, and brings the armature with it by the stop described. As soon as the valve closing member comes into contact with the valve seat, the closing movement of the valve needle is abruptly halted. The movement of the armature, which is not rigidly connected to the valve needle, continues against the stroke direction and is halted by the second return spring, in other words the armature follows through against the second return spring which has a much lower spring constant than the first return spring. Finally, the second return spring accelerates the armature back in the stroke direction.
One disadvantage with the fuel injector known from German Laid-Open Patent DE-OS 33 14 899 is the incomplete elimination of bounce, and on the other hand the arrangement of the armature and valve needle also makes it possible for the latter to tilt or stick as a result of center offset between the valve needle and the armature. This defect is intensified by manufacturing errors in the individual components of the fuel injector, leading to malfunctions of the injector.
In this connection it has also been suggested in U.S. Pat. No. 5,295,776 that the armature should not be connected rigidly to the valve needle, but that a certain axial play in the armature relative to the valve needle should be permitted.
The fuel injector shown in U.S. Pat. No. 5,299,776, however, is equipped with a flat armature, which is not guided within the injector housing but moves freely along the internal pole of the solenoid. In addition, the valve needle has only one guide sleeve, upon which the return spring is supported. A lower guide function is provided by a guide unit which is connected to the injector housing, with this guide unit surrounding the valve needle but not being connected to it in a friction locking manner.
The particular disadvantage of this arrangement lies in the restriction of the degree of freedom in the movement of the valve needle through the guide sleeve joined with the injector housing and thus in the danger of the valve needle tilting. Countering this disadvantage requires components that are manufactured extremely accurately, and these are characterized by high cost and very complex manufacture.
The fuel injector according to the present invention with the distinguishing characteristics of claim 1 has the advantage relative to the related art on the one hand that the radial and axial play of the valve needle brought about by the two guide sleeves and by the central opening in the armature provide so much freedom of movement that tilting is impossible, and on the other that the individual components of the fuel injector can be manufactured with a low degree of complexity, and low production costs, for example by deep drawing, since the design according to the present invention presents a very high tolerance for manufacturing errors in the components.
By the further measures listed in the dependent claims, advantageous further developments of the fuel injector described in the main claim are possible.
Also advantageous is the wedge-shaped or spherical design of the guide sleeves, and the corresponding elevations in the faces of the armature, which compensate for angular misplacements of the valve needle relative to the longitudinal axis of the fuel injector.
In addition, the symmetrical design, i.e. the rotatable mounting of the valve needle in the sealing seat, is advantageous, since this means that even in the event of major center offsets the valve needle can always align itself optimally.
Through the gaps between the guide sleeves and the armature, in addition, a slight pre-acceleration of the valve needle can be achieved, before the armature lifts the valve needle off the sealing seat. By this means the opening times or the amounts of fuel metered can be improved.